Alignment layer and liquid crystal display having the same

ABSTRACT

Disclosed are an alignment layer and a liquid crystal display having the same. The alignment layer comprises a polyimide compound having an azo group and exhibiting optical alignment characteristics and a discotic liquid crystal compound exhibiting optical anisotropy. The liquid crystal display comprises first and second substrates facing each other, liquid crystal aligned between the first and second substrates, and an alignment layer formed on at least one surface of the first and second substrates. The alignment layer is adjacent to the liquid crystal and has optical alignment characteristics and optical anisotropy.

CROSS-REFERENCE TO RELATED APPLICATION

This application relies for priority upon Korean Patent Application No.2006-107936 filed on Nov. 2, 2006, the contents of which are hereinincorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an alignment layer and a liquid crystaldisplay having the same. More particularly, the present inventionrelates to an alignment layer having optical alignment characteristicsand a liquid crystal display capable of widening a viewing angle byusing the alignment layer.

2. Description of the Related Art

A liquid crystal display displays an image by using liquid crystal. Theliquid crystal display includes a liquid crystal panel that includes twosubstrates and a liquid crystal layer interposed between the twosubstrates. The liquid crystal is aligned in the liquid crystal layer,and the liquid crystal has a refractive anisotropy so that thetransmittance of the light passing through the liquid crystal layervaries according to the alignment direction of the liquid crystal.

The alignment direction of the liquid crystal is adjusted by analignment layer formed on the two substrates. In order to adjust thealignment direction of the liquid crystal, a rubbing process isperformed. That is, a roller covered with cloth is rolled over thealignment layer to rub the alignment layer with the cloth. However, therubbing force may be non-uniform according to the position of thealignment layer during the rubbing process, so the liquid crystal may beirregularly aligned.

Because the liquid crystal has the refractive anisotropy, the quality ofthe image displayed in the liquid crystal display varies according tothe direction of the viewer. In general, the viewer usually watches theimage in front of the liquid crystal display, so the liquid crystaldisplay is operated to display the high quality image in the frontaldirection. However, as the viewer moves to the side of the liquidcrystal display, the image becomes distorted so that the image qualityis degraded. Thus, the liquid crystal display has a viewing angle, whichis a range of angles allowing the viewer to view the image clearly,narrower than that of other display apparatuses.

SUMMARY OF THE INVENTION

Therefore, the present invention provides an alignment layer whichoptically aligns liquid crystal to widen a viewing angle.

The present invention also provides a liquid crystal display having thealignment layer.

In one aspect, an alignment layer comprises a first polyimide compoundwhich is formed from a discotic liquid crystal compound and a secondpolyamide compound having an azo group.

In another aspect, a liquid crystal display comprises first and secondsubstrates facing each other, liquid crystal aligned between the firstand second substrates and an alignment layer formed on at least onesurface of the first and second substrates which are adjacent to theliquid crystal. The alignment layer comprises a first polyimide compoundwhich is formed from a discotic liquid crystal compound and a secondpolyimide compound having an azo group.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other advantages of the present invention will becomereadily apparent by reference to the following detailed description whenconsidered in conjunction with the accompanying drawings wherein:

FIG. 1 is a sectional view illustrating a liquid crystal displayaccording to an embodiment of the present invention;

FIG. 2 is an enlarged perspective view illustrating a part of a liquidcrystal panel shown in FIG. 1;

FIG. 3 is a view illustrating the process of interconversion betweentrans-azobenzenoid and cis-azobenzenoid by the light in an alignmentlayer shown in FIG. 2;

FIG. 4A is a view illustrating the process of widening a viewing anglein an alignment layer and a liquid crystal layer shown in FIG. 2;

FIG. 4B is a view illustrating the principle of compensation for a phasedifference between nematic liquid crystal and discotic liquid crystalshown in FIG. 4A; and

FIG. 5 is a view illustrating the process of optically aligning analignment layer shown in FIG. 2.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, embodiments of the present invention will be explained indetail with reference to the accompanying drawings. However, the scopeof the present invention is not limited to such embodiments and thepresent invention may be realized in various forms. The embodiments tobe described below are nothing but the ones provided to bring thedisclosure of the present invention to perfection and assist thoseskilled in the art to completely understand the present invention. Thepresent invention is defined only by the scope of the appended claims.In addition, the size of regions shown in the drawings can be simplifiedor magnified for the purpose of clear explanation and may not benecessarily drawn to scale. Also, the same reference numerals are usedto designate the same elements throughout the drawings.

FIG. 1 is a sectional view illustrating a liquid crystal displayaccording to an embodiment of the present invention.

Referring to FIG. 1, first and second polarizers 10 and 20, and a liquidcrystal panel 100 are provided. The liquid crystal panel 100 includesfirst and second substrates 110 and 120 that are facing each other, anda liquid crystal layer 140 interposed between the first and secondsubstrates 110 and 120. The first polarizer 10 is attached to the firstsubstrate 110 of the liquid crystal panel 100, and the second polarizer20 is attached to the second substrate 120 of the liquid crystal panel100.

The first polarizer 10 includes a first supporting film 11, a firstpolarizing film 12, a second supporting film 13 and a first protectingfilm 14. The first polarizing film 12 has a transmission axis in onedirection, and linearly polarizes incident light in a parallel directionto the transmission axis. The first and second supporting films 11 and13 are attached to both surfaces of the first polarizing film 12,respectively, to support the first polarizing film 12. The firstprotecting film 14 is attached to the second supporting film 13 at theoutermost side of the first polarizer 10 to protect the other films.

The second polarizer 20 has a structure corresponding to the firstpolarizer 10. The second polarizer 20 includes a second polarizing film22 that has the transmission axis perpendicular to that of the firstpolarizing film 12. Third and fourth supporting films 21 and 23 areattached to both surfaces of the second polarizing film 22, and a secondprotecting film 24 is attached to the top surface of the fourthsupporting film 23.

FIG. 2 is an enlarged perspective view illustrating a part of a liquidcrystal panel shown in FIG. 1.

Referring to FIG. 2, a plurality of gate lines 111 and data lines 112are formed on the first substrate 110. The gate lines 111 cross the datalines 112 to define pixel areas PA. Each pixel area includes a pixelelectrode 113 and a thin film transistor 115. The second substrate 120includes a common electrode 123 facing the pixel electrode 113. Firstand second alignment layers 131 and 132 are formed on the pixelelectrode 113 and the common electrode 123, respectively, while facingeach other.

Hereinafter, the operation of the liquid crystal display will bedescribed with reference to FIGS. 1 and 2.

Liquid crystal is aligned in the liquid crystal layer 140. The liquidcrystal has an oval shape having a long-axis and a short-axis, and analignment direction thereof is defined by the direction of thelong-axis. When voltage is not applied to the pixel electrode 113 andthe common electrode 123 during the operation of the liquid crystaldisplay, the liquid crystal is aligned parallel to the first and secondsubstrates 110 and 120. In addition, the liquid crystal on the firstsubstrate 110 and the liquid crystal on the second substrate 120 areperpendicular to each other, and the liquid crystal between the twosubstrates is continuously twisted. With such a state, light is suppliedand is linearly polarized as it passes through the first polarizer 10.As the linearly polarized light passes through the liquid crystal layer140, the phase thereof changes according to the twisted liquid crystal.The phase changed light passes through the second polarizer 20 todisplay the image.

Data voltage is applied to the pixel electrode 113 during the operationof the liquid crystal display. In addition, a common voltage is appliedto the common electrode 123. An electric field derived from the voltagedifference between the data voltage and the common voltage isestablished between the first and second substrates 110 and 120. Theliquid crystal has a dielectric anisotropy, and the liquid crystal isaligned perpendicular to the first and second substrates 110 and 120 asthe electric field is applied thereto. With such a state, the light issupplied and the light is linearly polarized as it passes through thefirst polarizer 10. The linearly polarized light is not subject to thephase change while passing through the liquid crystal layer 140, so thelight does not pass through the second polarizer 20. Thus, the liquidcrystal display becomes a black state.

The liquid crystal may be inclined relative to the first and secondsubstrates 110 and 120 according to the intensity of the electric fieldapplied thereto. The phase change of the light is incurred according tothe inclination angle, and some light passes through the secondpolarizer 20 so that the image having a gray scale may be displayed.

In such an operation, the alignment direction of the liquid crystal isadjusted by the first and second alignment layers 131 and 132 when theelectric field is not applied to the liquid crystal. The first andsecond alignment layers 131 and 132 have optical alignmentcharacteristics through a photo-isomerization reaction. The first andsecond alignment layers 131 and 132 include a polyimide compound havingan azo group in order to have the optical alignment characteristics.

The polyimide is a polymer compound having an imide group (CO—N—CO), andrepresents superior stability, durability and productivity. The azogroup (N═N) includes double-bonded nitrogen atoms, and has trans/cisisomers due to the double-bonded nitrogen atoms. The isomers exhibit thephoto-isomerization reaction which is the interconversion between thetrans/cis isomers by the light. The liquid crystal can be aligned in adesired direction through the photo-isomerization reaction.

A compound represented by chemical formula 1 as follows can be used asthe polyimide compound having the azo group.

The compound of chemical formula 1 is a polymer compound having centralhydrocarbon that is capped with an imide group and azobenzenoid, and hasacrylic groups

bonded to the terminal ends thereof.

FIG. 3 is a view illustrating the process of interconversion betweentrans-azobenzenoid and cis-azobenzenoid by the light in an alignmentlayer shown in FIG. 2.

Referring to FIG. 3, the azobenzenoid is more stable in the trans isomerthan the cis isomer. The trans-azobenzenoid chain is alignedperpendicular to the alignment layer 130. The light is supplied to theentire surface of the alignment layer 130 such that thetrans-azobenzenoid absorbs the light. Then, the trans-azobenzenoid isconverted into the cis-azobenzenoid through the photo-isomerizationreaction. The cis-azobenzenoid chain is aligned parallel to thealignment layer 130. Since the cis-azobenzenoid is unstable, it isimmediately converted into the trans-azobenzenoid by a thermalexcitation, so that azobenzenoid can be aligned in the directionperpendicular to the alignment layer 130. In this case, the azobenzenoiddoes not absorb light and can be stabilized.

Through such a process, the alignment layer including the azobenzenoidhas a stronger structure, in which the mobility of liquid crystal isreduced as compared with the state before the alignment layer isstabilized. Therefore, the alignment layer 130 imposes a strongelasticity to the liquid crystal, and the liquid crystal can be alignedin the direction perpendicular to the alignment layer 130 according tothe direction of the trans-azobenzenoid.

Meanwhile, the light can be supplied to the alignment layer 130 whileforming a specific angle relative to the alignment layer 130. In thiscase, the azobenzenoid can be converted into the trans isomer to bestabilized through the photo-isomerization reaction, in which the transisomer is inclined relative to the alignment layer 130 at a specificangle. The liquid crystal is inclined according to the direction of theinclined trans-azobenzenoid. The liquid crystal can be horizontallyaligned at a specific linear inclination angle by adjusting the incidentangle of the light.

The alignment layer 130 can have alignment characteristics through arubbing process as well as supplying the light thereto. That is, aroller covered with cloth is rolled over the alignment layer 130 to rubthe alignment layer 130 with the cloth such that the alignment layer 130may have the alignment characteristics. However, the rubbing force maybe non-uniform according to the position of the alignment layer duringthe rubbing process, so the liquid crystal may be irregularly aligned.In contrast, the light can be uniformly supplied on the entire surfaceof the alignment layer 130 through the process of forming the alignmentby the light, so the process according to the present embodiment is moreefficient than the rubbing process.

The alignment layer 130 according to the present embodiment has anoptical anisotropy as well as the optical alignment characteristics, andthe viewing angle of the liquid crystal display is widened by suchcharacteristics of the alignment layer 130.

FIG. 4A is a view illustrating the process of widening a viewing anglein an alignment layer and a liquid crystal layer shown in FIG. 2, andFIG. 4B is a view illustrating the principle of compensation for a phasedifference between nematic liquid crystal and discotic liquid crystalshown in FIG. 4A.

Referring to FIG. 4A, the alignment layer 130 includes discotic liquidcrystal 160 having the optical anisotropy. Various liquid crystals maybe used in the liquid crystal layer 140 according to the operationalprocedure of the liquid crystal display. For instance, nematic liquidcrystal 150 corresponding to the discotic liquid crystal 160 may beused. The nematic liquid crystal 150 has a bar shape, and the discoticliquid crystal 160 has a disk shape.

The liquid crystal layer 140 includes a twisted structure in which thenematic liquid crystal 150 is positioned between the first and secondalignment layers 131 and 132 and the alignment direction of the nematicliquid crystal 150 is twisted. In this case, the image may be distortedat a lateral direction which means an inclined direction with regard tothe second alignment layer 132 due to a refractive anisotropy of thenematic liquid crystal 150. However, the distortion of the image in thelateral direction can be prevented by the discotic liquid crystal 160.

Referring to FIG. 4B, the nematic liquid crystal 150 is a positiveuniaxial medium having the bar shape, so it has an extraordinaryrefractive index larger than an ordinary refractive index. The discoticliquid crystal 160 is a negative uniaxial medium having the disk shape,so it has an extraordinary refractive index smaller than an ordinaryrefractive index. Therefore, as shown in FIG. 4B, if the nematic liquidcrystal 150 and the discotic liquid crystal 160 are aligned parallel tothe traveling direction of the light, the phase difference according tothe traveling direction of the light can be compensated.

As shown in FIG. 4A, the nematic liquid crystal 150 is continuouslytwisted in the liquid crystal layer 140. The twisted degree variesaccording to the vertical location of the nematic liquid crystal 150 inthe liquid crystal layer 140. The discotic liquid crystals 160 arealigned such that it corresponds to each of the nematic liquid crystal150 having different twisted degrees. When dividing the liquid crystallayer 140 into two portions in the parallel direction, the discoticliquid crystals 160 in the first alignment layer 131 correspond to thenematic liquid crystals 150 in the lower portion of the liquid crystallayer 140, and the discotic liquid crystals 160 in the second alignmentlayer 132 correspond to the nematic liquid crystals 150 in the upperportion of the liquid crystal layer 140.

As described above, the alignment layer 130 includes a discotic liquidcrystal compound, so the image quality is improved in the lateraldirection. As a result, the viewing angle of the liquid crystal displayis widened. A separate film may be made by using the discotic liquidcrystal compound, and the film can be included in the first and secondpolarizers 10 and 20. In this case, the viewing angle of the liquidcrystal display can also be widened, but the separate film must bemanufactured. Thus, the method of the present embodiment employing thealignment layer 130 is more efficient than the process employing theseparate film.

A compound represented by chemical formula 2 as follows can be used asthe compound of the discotic liquid crystal 160.

The compound of chemical formula 2 has a structure of tripenylene, andhas four acrylic groups

bonded to the terminal ends thereof. The polyimide compound of chemicalformula 1 having the azo group and the discotic liquid crystal compoundof chemical formula 2 react with each other by light. During thereaction, the acrylic groups included in the terminal ends of chemicalformulas 1 and 2 react with each other to incur a radical photo-curing.As a result, a compound having chemical formula 3 as follows is formed.

The compound of chemical formula 3 is a polyimide compound, and has theazo group at the main chain thereof to exhibit the optical alignmentcharacteristics. In addition, the compound of chemical formula 3 has theoptical anisotropy due to the discotic liquid crystal compound which isbonded to the terminal end thereof.

Meanwhile, the alignment layer 130 is derived from a polyimide compoundhaving chemical formula 4 as follows and the discotic liquid crystalcompound having chemical formula 5 as follows.

The compound of chemical formula 4 is a polymer having the imide groupand the azobenzenoid bonded to both ends of the central hydrocarbon,respectively, and has epoxy groups

bonded to the terminal ends thereof. The compound of chemical formula 5has a structure of tripenylene, and has four epoxy groups

bonded to the terminal ends thereof. The polyimide compound of chemicalformula 4 having the azo group and the discotic liquid crystal compoundof chemical formula 5 react with each other by light.

During the reaction, the epoxy groups included in the terminal ends ofchemical formulas 4 and 5 react with each other to incur a cationphoto-curing. As a result, a compound having chemical formula 6 asfollows is formed.

The compound of chemical formula 6 is a polyimide compound, and has theazo group at the main chain thereof to exhibit the optical alignmentcharacteristics. In addition, the compound of chemical formula 6 has theoptical anisotropy due to the discotic liquid crystal compound which isbonded to the terminal end thereof.

Meanwhile, the alignment layer 130 can be formed from a polyimidecompound having chemical formula 7 as follows and the discotic liquidcrystal compound having chemical formula 5.

The compound of chemical formula 7 is a polymer having the imide groupand the azobenzenoid bonded to both ends of the central hydrocarbon,respectively, and has amine groups (NH₂) bonded to the terminal endsthereof. The polyimide compound of chemical formula 7 having the azogroup and the discotic liquid crystal compound of chemical formula 5react with each other by heat.

During the reaction, the amine group included in the terminal end ofchemical formula 7 and the epoxy group included in the terminal end ofchemical formula 5 react with each other to incur a thermosetting. As aresult, a compound having chemical formula 8 as follows is formed.

The compound of chemical formula 8 is a polyimide compound, and has theazo group at the main chain thereof to exhibit the optical alignmentcharacteristics. In addition, the compound of chemical formula 8 has theoptical anisotropy due to the discotic liquid crystal compound which isbonded to the terminal end thereof.

Meanwhile, the alignment layer 130 can be formed from a polyimidecompound having chemical formulas 4 and 7 and the discotic liquidcrystal compound having chemical formula 5. The compound of chemicalformula 4 and the compound of chemical formula 5 react with each otherby light, and as a result, the compound having chemical formula 6 isformed. The compound of chemical formula 7 and the compound of chemicalformula 5 react with each other by heat, and as a result, the compoundhaving chemical formula 8 is formed. The compound having chemicalformula 6 and the compound having chemical formula 8 are polyimidecompound, and both compound have the azo group at the main chain thereofto exhibit the optical alignment characteristics. In addition, thecompound having chemical formulas 6 and the compound having chemicalformula 8 have the optical anisotropy due to the discotic liquid crystalcompound which is bonded to the terminal ends thereof.

As described above, the alignment layer 130 includes at least one of thecompounds of chemical formula 3, 6 and 8 through the photo-curing or thethermosetting. The curing process is rapidly performed in thephoto-curing, so the liquid crystal layer 140 is prevented from beingcontaminated by the alignment layer 130 before the curing process.However, the region where no light reaches the liquid crystal panel 100may be non-cured during the photo-curing. In contrast, although thecuring speed in the thermosetting is slow, the curing is uniformlyperformed throughout the liquid crystal panel 100.

Various advantages as well as improvement of the viewing angle of theliquid crystal display can be achieved during the manufacturing processfor the liquid crystal display by using the alignment layer 130.Hereinafter, the various advantages will be described with reference tothe process of manufacturing the liquid crystal display by using theabove-mentioned alignment layer material.

Referring again to FIG. 2, the first and second substrates 110 and 120are separately prepared.

In the first substrate 110, a gate conductive layer is formed on thefirst substrate 110, and then, the gate conductive layer is patterned toform the gate lines 111 and gate electrodes. After forming a dataconductive layer on the gate lines 111, the data conductive layer ispatterned to form the data lines 112, source electrodes and drainelectrodes. The thin film transistor 115 is formed by each gateelectrode, each source electrode and each drain electrode. After forminga transparent conductive layer on the data lines 112, the transparentconductive layer is patterned to form the pixel electrode 113. The firstalignment layer 131 is formed on the pixel electrode 113. In the secondsubstrate 120, the transparent conductive layer is deposited on thesecond substrate 120 to form the common electrode 123. The secondalignment layer 132 is formed on the common electrode 123.

The compound including the polyimide compound having the azo group andthe discotic liquid crystal compound is melted in appropriate solvent,thereby forming a solution. After coating the solution on the pixelelectrode 113 and the common electrode 123, the solution is cured,thereby forming the first and second alignment layers 131 and 132.

FIG. 5 is a view illustrating the process of optically aligning analignment layer shown in FIG. 2. Although FIG. 5 shows the process ofoptically aligning the second alignment layer 132, this can also beapplicable for the first alignment layer 131.

Referring to FIG. 5, a light source 210, a polarizer 220 and a stage 230are provided. The second substrate 120 is placed on the stage 230. Light300 generated from the light source 210 is linearly polarized whilepassing through the polarizer 220. The linearly polarized light 300′ issupplied to the surface of the second alignment layer 132 while having apredetermined angle (θ) inclined relative to the surface of the secondalignment layer 132. The alignment direction of the liquid crystal isadjusted according to the angle (θ). For instance, when the angle (θ) isabout 45 degrees, the liquid crystal can be aligned to have a pretiltangle of about 5 degrees.

As described above, the rubbing process can be performed relative to thealignment layer 130 such that the alignment layer 130 may have thealignment characteristics. That is, the alignment layer 130 is rubbedwith the cloth. However, a pressure is applied to the alignment layer130 during the rubbing process, so that the pressure may exert aninfluence upon the discotic liquid crystal compound included in thealignment layer 130. As a result, the discotic liquid crystal isinappropriately aligned, thereby narrowing the viewing angle of theliquid crystal display. However, such a problem can be prevented byusing the optical alignment as described in the present embodiment.

According to the embodiment, the viewing angle of the liquid crystaldisplay is widened by using the alignment layer having the opticalanisotropy. In addition, the alignment characteristics can be easilyimparted to the alignment layer by the light.

Although embodiments of the present invention have been described, it isunderstood that the present invention should not be limited to theseembodiments but various changes and modifications can be made by oneordinary skilled in the art within the spirit and scope of the presentinvention as hereinafter claimed.

1. An alignment layer comprising a first polyimide compound that isformed from a second polyimide compound having an azo group and adiscotic liquid crystal compound.
 2. The alignment layer of claim 1,wherein the second polyimide compound is represented by

, and the discotic liquid crystal compound is represented by


3. The alignment layer of claim 2, wherein the first polyimide compoundhas a chemical formula as follows:


4. The alignment layer of claim 1, wherein the second polyimide compoundis represented by

, and the discotic liquid crystal compound is represented by


5. The alignment layer of claim 4, wherein the first polyimide compoundhas a chemical formula as follows:


6. The alignment layer of claim 1, wherein the second polyimide compoundis represented by

, and the discotic liquid crystal compound is represented by


7. The alignment layer of claim 6, wherein the first polyimide compoundhas a chemical formula as follows:


8. The alignment layer of claim 1, wherein the second polyimide compoundincludes a compound represented by

, and the discotic liquid crystal compound is represented by


9. The alignment layer of claim 8, wherein the first polyimide compoundincludes a compound represented by


10. A liquid crystal display, comprising: first and second substratesfacing each other; liquid crystal aligned between the first and secondsubstrates; and an alignment layer formed on at least one surface of thefirst and second substrates to be adjacent to the liquid crystal,wherein the alignment layer comprises a first polyimide compound that isformed from a second polyimide compound having an azo group and adiscotic liquid crystal compound.
 11. The liquid crystal display ofclaim 10, wherein the second polyimide compound is represented by

, and the discotic liquid crystal compound is represented by


12. The liquid crystal display of claim 11, wherein the first polyimidecompound has a chemical formula as follows:


13. The liquid crystal display of claim 10, wherein the second polyimidecompound is represented by

, and the discotic liquid crystal compound is represented by


14. The liquid crystal display of claim 13, wherein the first polyimidecompound has a chemical formula as follows:


15. The liquid crystal display of claim 10, wherein the second polyimidecompound is represented by

, and the discotic liquid crystal compound is represented by


16. The liquid crystal display of claim 15, wherein the first polyimidecompound has a chemical formula as follows:


17. The liquid crystal display of claim 10, wherein the second polyimidecompound is represented by

, and the discotic liquid crystal compound is represented by


18. The liquid crystal display of claim 17, wherein the first polyimidecompound includes a compound represented by


19. The liquid crystal display of claim 10, wherein the alignment layeris supplied by a linearly polarized light which is inclined relative tothe alignment layer to exhibit the optical alignment characteristics.20. The liquid crystal display of claim 19, wherein the liquid crystalincludes nematic liquid crystal.